Silicon carbide (SiC) is a wide-bandgap semiconductor having a broad forbidden band width of 2.2 to 3.3 eV. Owing to its outstanding physical and chemical properties, SiC has become a focus of research and development for its potential as an environmentally rugged semiconductor material. In recent years, moreover, SiC has attracted increasing attention as a material for short wavelength optical devices in the blue-to-UV spectral region, high-frequency electronic devices, high-voltage electronic devices, high output electronic devices, and the like. Although R&D is being aggressively pursued with regard to such applications, production of large-diameter single-crystal silicon carbide of high quality has so far proved to be difficult to achieve, and this has thwarted practical use of SiC devices.
Single-crystal SiC of a size usable for fabrication of semiconductor devices has up to now been conducted on a laboratory scale using, for example, the sublimation recrystallization process (Lely process). However, the single crystal obtained by this method is of small area, and its dimensions and shape are difficult to control. Moreover, control of the crystal polytype and doping carrier concentration of the SiC is not easy either. On the other hand, cubic single-crystal SiC is being produced by heteroepitaxial growth, i.e., growth on a substrate of a different type like silicon (Si), using chemical vapor deposition (CVD). Although large-area single crystal can be obtained by this process, high-quality single-crystal SiC cannot be obtained because, among other reasons, the approximately 20% lattice-mismatch between the SiC and Si invariably causes growth of SiC single crystal containing many defects (˜107/cm2).
The modified Lely process, which conducts sublimation recrystallization using a single-crystal SiC wafer as a seed, was developed to overcome these problems (Non Patent Literature 1). The modified Lely process makes it possible to grow single-crystal SiC while controlling its polytype (6H, 4H, 15R and other polytypes), shape, and carrier type and concentration.
Currently, 51 mm (2-inch) to 100 mm single-crystal SiC wafers are being cut from single-crystal SiC produced by the modified Lely process and used, for example, to fabricate devices in the power electronics field. In most cases, however, the crystals are observed to contain micropipes (hollow hole-like defects extending in the longitudinal direction of the crystal) at a density of up to around several tens cm−2 and dislocation defects at a density on the order of 104 to 105 cm−2. As pointed out, for example, in Non Patent Literature 2, and Non Patent Literature 3, these defects cause leakage current and other problems in a fabricated electronic device. Crystal defect reduction is considered one of the most important issues in the application of single-crystal SiC in devices.
Numerous reports have been published on research into micropipes, which are viewed as the typical defect of single-crystal SiC. The micropipes in the grown crystal are often ones inherited from micropipes present in the seed crystal. Although the average number of micropipes present in a single-crystal SiC wafer has declined with advances in crystal growth technology, a complete solution has not yet been found to the problem of the number of micropipes in the grown crystal being greater than in the seed crystal owing to new micropipes arising from starting points that are secondary phases such as foreign polytypes, polycrystal and the like included during crystal growth.
In recent years, the dislocation defects of single-crystal SiC have also become a major focus of attention. Although many aspects of the occurrence and propagation of dislocation defects in single-crystal SiC are not fully understood, the general situation can be summarized as follows.
Threading dislocations like threading screw and edge dislocations resemble micropipes in the point that they are often inherited by the grown crystals from ones present in the seed crystal from the start. Still, one property of single-crystal SiC is that dislocations whose slip plane is the basal plane ({0001})(basal plane dislocations) occur relatively easily during crystal growth. This is because in the modified Lely process, the classic method of producing single-crystal SiC, occurrence of thermal stress is substantially unavoidable and the critical sheer stress of the {0001} plane that is the main SiC slip plane is very small at high temperature (see, for example, Non Patent Literature 4). As the basal plane dislocation itself has a slip line substantially perpendicular to the growth direction, it does not propagate in the growth direction. But in the course of crystal growth, the basal plane dislocation may change to and be inherited as a threading dislocation having a slip line passing through in the growth direction ([0001]), which is thought to increase the dislocation density of the grown crystal.
As explained in the foregoing, although the quality of the grown crystal is strongly dependent on the quality of the seed crystal, it is also a fact that, even when the crystal is grown using high-quality single-crystal SiC as the seed crystal, the quality of the grown crystal is nevertheless often degraded by the occurrence of new crystal defects owing to inclusion of foreign polytypes and polycrystal during crystal growth, as well as to unavoidable thermal stress and other factors. Considerable research has been focused on the development of technologies for overcoming this problem and enabling stable production of high-quality single-crystal SiC.
For example, Patent Literature 1 teaches a method for stable growth of a desired polytype by adding a certain kind of dopant. This technology is aimed at stable production of 4H polytype by preferential nucleation under high C/Si ratio. The carbon/silicon element ratio (C/Si ratio) in the crystal is effectively increased during single-crystal SiC growth by adding nitrogen at carbon atom sites at an atomic number density of 5×1018 cm−3 or greater, preferably 5×1019 cm−3 or greater.
Patent Literature 2 teaches that, where N number (N=natural ordinal number not less than 3) of growth steps are incorporated and defined as nth growth steps (n=natural ordinal number between one and N, inclusive), the first growth step (n=1) uses a first seed crystal wherein a surface of an offset angle of ±20° or less from the {1-100} plane or a surface of an offset angle of ±20° or less from the {11-20} plane is exposed as a first growth surface on which single-crystal SiC is grown to prepare a first grown crystal, the intermediate growth steps n=2, 3 . . . , (N−1) each prepares from an (n−1)th seed crystal an nth seed crystal wherein the nth growth surface is a surface inclined from the (n−1)th growth surface by 45 to 90° and from the {0001} plane by 60 to 90° and grows single-crystal SiC on the nth growth surface of the nth seed crystal to prepare nth grown crystal, and the final growth step (n=N) prepares from the (N−1)th grown crystal a final seed crystal wherein a surface of an offset angle of ±20° or less from the {0001} plane of the (N−1)th grown crystal is exposed as a final growth surface and bulk SiC crystal is grown on the final growth surface of the final seed crystal, thereby establishing a method of producing high-quality single-crystal SiC with few micropipe defects, dislocation defects, stacking faults and the like.
On the other hand, the problems that have a major effect on the quality of the single-crystal SiC most often arise at the initial stage of crystal growth. Non Patent Literature 5, for example, report a phenomenon of heavy occurrence of dislocation defects at the very start of crystal growth, i.e., at the interface between the seed crystal and the grown crystal. In addition, it is known from Non Patent Literature 6, for example, that the probability of foreign polytype occurrence is high at the start of crystal growth.
Some of the defects occurring at the start of crystal growth disappear during the ensuing crystal growth, so that defect density decreases toward the latter half of the growth. But since some remain to the end of the crystal growth, the defect density of the grown crystal does not decrease below a certain level no matter how high the quality of the seed crystal used. Inhibition of crystal defect occurrence at the start of growth is therefore indispensable for realizing high single-crystal SiC quality.
Although the reasons for crystal defects occurring at the start of growth have not all been determined, one is probably that the thermal stress unavoidably produced in the modified Lely process is large at the interface between the seed crystal and the grown crystal. Moreover, recent studies have found that another major cause is the difference in doping element density between the seed crystal and the grown crystal.
The invention of Patent Literature 3 was made with focus on doping element concentration difference. This publication teaches a method of producing high-quality single-crystal silicon carbide by gradually increasing or gradually decreasing additive element concentration in the growth crystal within a predetermined range of concentration change from the same concentration as that of the seed crystal to a desired concentration, thereby inhibiting defect occurrence at the interface between the seed crystal and the grown crystal.